Process for preparing crystalline sodium nitriloacetic acid

ABSTRACT

It is disclosed that the particle size distribution of crystalline sodium nitrilotriacetate (NTA) can be controlled to achieve a desired bulk density of the product and to provide a product that centrifuges readily. The control is obtained by conducting the crystallization and/or crystal growth of NTA in the presence of a centrifugal impeller operative at a peripheral velocity of from about 20 to about 80 feet per second whereby the crystals are selectively abraded or fractured to limit growth of the periphery of the crystals enhancing the relative growth rate of the crystals in the thickness dimension.

United States Patent n 1 Agee et al.

1 Sept. 11, 1973 PROCESS FOR PREPARING CRYSTALLINE SODIUM NITRILOACETICACID [75] lnventors: Robert B. Agee, Baker; William R.

Cooke, Baton Rouge, both of La.

[73] Assignee: Ethyl Corporation, Richmond, Va.

[22] Filed: Dec. 7, 1970 [21] Appl. No.: 95,540

[52] US. Cl. 260/534 E [51] Int. Cl. C07c 101/20 [58] Field of Search260/543 E; 23/295 [56] References Cited UNITED STATES PATENTS 3,218,13311/1965 Ebner 23/295 3,399,976 9/1968 Randolph 23/295 OTHER PUBLICATIONSBamforth industrial Crystallization, Leonard Hill (1965) pp. 156-167.

Primary Examiner-Lorraine A. Weinberger Assistant Examiner-John F.Terapane At!0mey-D0nald L. Johnson, John F. Sieberth, Shelton B.McAnelly and Arthur G. Connolly [57] ABSTRACT It is disclosed that theparticle size distribution of crystalline sodium nitrilotriacetate (NTA)can be controlled to achieve a desired bulk density of the product 14Claims, 1 Drawing Figure Patented Sept. 11, 1973 rlllllllllllllllllll Q05 580% v E om l l l 193m mwh oz] 000 PROCESS FOR PREPARING CRYSTAL-LINESODIUM NITRILOACETIC ACID BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to the preparation of crystallinesodium nitrilotriacetate (NTA) by recovery from a supersaturatedsolution.

2. Description of the Prior Art Several processes are available forrecovering solid NTA from an aqueous solution thereof. One such processis by crystallization from a supersaturated solution followed byseparation of the resulting crystals in a centrifugal device to recoverthe crystals substantially free of mother liquor. It is characteristicof such crystallization that crystals formed are thin platelets whichare characterized by a low bulk density of the recovered crystal productand by difficulty of separation of the crystals from the mother liquorin centrifugal devices because the crystals interweave. Thesecharacteristics are not significantly altered even with slow growth tocomparatively large crystals or by other variations of the rate ofcrystal growth through such expedients as temperature and concentrationvariations of the growing solution because the ratio of crystalthickness to maximum periphery is unimpeded crystal growth issubstantially constant.

SUMMARY OF THE INVENTION It has been discovered that an improvement ispossible in the growth of NTA crystals to produce thicker crystalshaving a larger numerical ratio of thickness to periphery than thatcharacterized by ordinary growth by mechanically agitating the mixtureof growing crystals and accompanying mother liquor. With the properseverity of agitation as defined herein, the crystals are broken in aplane parallel to the thickness dimension without serious abrasion ofthe thickness dimension or retardation of the growth in the thicknessdimension. The result is that the crystals are allowed to build upselectively in the thickness dimension with growth in directions at'right angles to the thickness dimension being hindered. The product fromsuch a crystallization operation, although frequently apparently smallerin size than the ordinary product when measured on a basis of meshscreens, centrifuges readily and has a desirable high bulk density ofthe order of 0.5 to 0.8 grams per cc.

Controlled manipulation of the crystals in the growth process ispreferably accomplished with an agitation device which provides amoderate degree of shear. A centrifugal pump of conventional design withclosed or open impeller when operated at certain speeds is an example ofsuch a moderate shear agitation device. When such a pump is operatedwithin the crystal growing system using peripheral velocities of thepump impeller of from about 20 to about 80 feet per second desirableresults with respect to control of the size and shape of the producttrisodium nitrilotriacetate (NTA) are obtained. In this connection, itis noteworthy that the intimacy of contact attainable with a centrifugalimpeller is apparently an important factor since other types of pumpscommonly encountered such as the axial flow propeller type wherein theflow is primarily axial rather than centrifugal, do not produce thepresently desired results even when operated at the upper regions of theperipheral velocities recited in the foregoing. On the other hand, geartype and similar positive displacement pumps, even when operated at lowspeeds, exert a crushing and grinding action between internal surfacesproviding generally a locally excessive comminution 5 action.

Although the preferred manner of use of the centrifugal impeller iswithin a centrifugal pump (or a turbine pump) housing in a pipedrecirculation system, the present desired results are also obtained whensuch an 0 impeller is merely inserted into a mass of concentratedaqueous NTA solution and NTA crystals and rotated so as to provide thespecified peripheral velocities. In general, simple propeller typeagitators inserted within the mass provide an equivalent of axial flowrather than centrifugal or radial flow and do not provide the presentlydesired results in a satisfactory manner.

The present invention relates to a process for producing solidparticulate NTA of high bulk density by crystallization from asupersaturated aqueous solution of NTA favorable to the growth of thinplatelet NTA crystals. The system of supersaturated solution andcrystals is mechanically agitated, while the crystals are growing, bycirculation through a centrifugal impeller operative at a peripheralvelocity of from about to about 80 feet per second whereby the crystalsare selectively abraded or fractured to effectively limit growth of theperiphery of the crystals enhancing the relative growth rate of thecrystals in the thickness dimension.

In preferred aspects, the centrifugal impeller peripheral velocity isfrom about to about 70 feet per second, a more preferred range beingfrom about to about feet per second. By way of illustration, a typicalperipheral velocity of the centrifugal impeller is about 50 feet persecond.

In a preferred aspect, the centrifugal impeller is the impeller of acentrifugal pump disposed within a pump housing forming a pump which isinterconnected by way of piping whereby the entire volume of the systemof supersaturated aqueous solution of NTA and NTA crystals circulatesthrough the pump in a period of from about 0.5 to about 5 minutes. Atypical volumetric flow circulation rate for thesystem through the pumpis an average of about once every 2 minutes.

In a preferred aspect of the process of the present invention, thesystem of NTA solution and NTA crystals is maintained at a temperatureof from about l20 to about 250F. A more preferred temperature range isfrom about l50 to about 200F, or even from about l to about 195F. Atypical preferred temperature is lF. A typical temperature in accordancewith the present invention is about F.

In a preferred aspect, the process of the present invention grows NTAcrystals in a system consisting essentially of an aqueous solution ofNTA in admixture with from about 5 to about 50 wt. percent NTA crystals.In a more preferred aspect of the present invention, the crystal growingis conducted in a system which consists essentially of an aqueoussolution of NTA and from about 20 to about 40 wt. percent NTA crystals.A typical system consists essentially of a mixture of an aqueoussolution of NTA with about 35 wt. percent NTA crystals and from about ito about 5 percent NaOH copresent.

The present process produces solid particulate NTA of high bulk densityand controlled particle size and shape by crystallization from anaqueous solution which contains from about 5 to about 50 wt. percent NTAcrystals in addition to at least a saturation amount of NTA at thetemperature of operation which is from about 120 to about 250F.Preferably, the solution is formed by the evaporation of water from afeed solution which is somewhat less than saturated. The foregoingsolution is agitated with a centrifugal impeller operative at a maximumperipheral velocity of from about 20 to about 80 feet per second wherebythe growing crystals are selectively abraded.

The present invention provides a process for producing solid particulateNTA of high bulk density and controlled particle size and shape whereinthe NTA is crystallized from a solution. An important aspect of theprocess for the purpose of controlling particle size and shape is therecirculation of a system consisting essentially of an aqueous solutionof NTA containingfrom about to about 50 wt. percent free NTA crystals ina heater-flash evaporator wherein the system is alternately heated to atemperature of about 200F and flashed at a temperature of about 195Fwhereby water vapor is removed. Feed to the system is an NTA solutionconsisting essentially of from about 30 to about 40 wt. percent NTA andwater. The recirculation of the process is conducted in two streamsincluding l a low shear stream operative at a volumetric flow rate perminute of from about 1 to about times the volume of said system, and (2)a moderate shear stream containing a centrifugal pump operative at animpeller peripheral velocity of from about 20 to about 80 feet persecond and at a volumetric flow rate per minute of from about 0.2 toabout twice the volume of said system.

The present invention provides aprocess for the concurrent concentrationand'crystallization of a solution of NTA ranging from about a saturatedsolution to a more dilute solution containing about 5 percent of thesaturation amount of NTA. In the'process there is performed a concurrentevaporation. of water to form a chemical system containing a solution ofNTA, usually supersaturated, and in addition, from about 5 to about 50percent by weight of NTA crsytals. The growth of crystals within thesystem is selectively controlled by a deliberate abrading or fracturingof the crystals by a centrifugal impeller operative within the system ata selected peripheral velocity. in a particularly preferred aspect, thepresent invention is performed in a mechanical system whereby thechemical system described in the foregoing is repetitively recirculatedthrough a heater and a flash chamber by means of a pump at a rate fromabout I to about 10 times per minute.

In preferred embodiments of the present invention in addition to therecirculation loop described in the foregoing, there is provided asecond loop through a pump containing a centrifugal impeller operativeat a peripheral velocity of from about 20 to about 80 feet per secondthrough which the volumetric contentsof the system flows in a period offrom about 0.5 to about 5 minutes. In preferred embodiments, the firstmentioned pump is a low shear axial flow pump with a propeller typeagitator and the centrifugal impeller pump is a centrifugal pump ofconventional design with closed impeller or open impeller construction.

An innovation of the process of the present invention is the deliberateoperation of an agitator element within a crystallization system ataspeed which is intended to cause controlled breaking of the crystals inthe system.

The process of the present invention has been employed with centrifugalimpeller pumps of various sizes ranging from a 4% inch diameter impelleroperative at 2,700 rpm providing 53 feet per second peripheral velocityto a I05; diameter impeller pump operative at a speed of 1,900 rpm toprovide 87 feet per second peripheral velocity. Larger and smallerdiameter imepllers are operated at proportionate speeds to attain theperipheral velocities set forth. Excellent operation is obtained withthe 10% inch diameter impeller operative at a speed of l,l60 rpmproviding 53 feet per second peripheral velocity of the impeller.Throughput rate is determined in a conventional way involvingproportioning of the diameter, speed, shape and thickness of theimpellers. Where desired, increased flows are obtained without increaseddiameters of peripheral speeds by paralleling one or more impellers.

Direct measurement of throughput rate is not conveniently accomplishedwhen the centrifugal impeller is located in the open in a large vessel.In this instance the actual flow across the impeller generally is oflarger volume with less severity per unit of volume passing through theimpeller. Also the horsepower requirements may increase.

It is evident that the procedures described herein provide a basis forroutine experimentation to tailor make the particle size, shape and bulkdensity of the product NTA using either the direct agitation or separatepump approach. a

With reference now to the drawing, the apparatus shown therein includesa flash chamber It), typically operative at a temperature of F and apressure of about one-half atmosphere absolute. Steam flashed in theflash chamber 10 passes to condenser 11 where it is condensed andremoved as water through line 12 with non-condensables flowing throughline 13 to a vacuum system 14.

The flash chamber 10 is connected to a first recirculation loop of line15, pump 16, heater 17 and lines 18, 19 and 20. This first recirculationloop is proportioned primarily to provide the desired heat input to thesystem for evaporation of water with a controlled warmup of about 4 to6F in the heater 17. In general, this requires a flow rate in the firstrecirculation loop of from about I to about 10 times the liquid volumeof the system per minute, typically about 2 times the volume circulatedper minute. The pump 16 is designed to provide the high flow rate at lowhead since the total head required to be produced by that pump is mainlyonly that introduced by friction in the lines 15, l8, l9 and 20 and inthe heater 17. Typically, this pump is an axial flow pump with apropeller" type of impeller similar to those used in marine service. Inmany instances these pumps attain peripheral velocities of 2080 feet persecond; however, the flow is rapid and much of the material flowsthrough slower moving portions of the impeller. Thus this contact is notadequate for the purposes of the present invention. In fact, such pumpsmay be deliberately designed and operated to avoid breaking or abradingthe crystals. Such pumps are of low shear type under the normaloperating conditions employed.

A second recirculation loop is provided within the system employing thepump 21 and the connecting line 22 togetherwith one or moreof thealternate connecting lines 23 and 25. The recirculating pump 21typically is a pump of a type which is considerably different from pump16 in that it is operated to produce a moderate shear action and issuitably of less volumetric throughput capacity than pump 16. Pump 21contains a centrifugal impeller operative at a peripheral velocity offrom about 20 to about 80 feet per second with a volumetric flow rate offrom about 0.2 to about twice the volume of said system per minute,typically about 0.5 volume of the system per minute. The alternatedischarge lines 23 and 25 for pump 21 provide for convenience ofconnection or to permit adding the volume of pump 21 to the volume ofpump 16 for the feed through heater 17. In general, the discharge of thepumps 16 and 21 are advantageously combined for feed through the heater17 in those instances where flow rate through pump 21 is greater thanabout half that of the flow rate through pump 16.

Feed to the system is provided at any convenient point, typically toline between the flash chamber 10 and the pump 16. The feed preferablyis an NTA solution which is about saturated containing about 1 percentresidual sodium hydroxide, a characteristic of a preferred method ofproduction of the feed stream which is typically by reaction ofnitrilotriacetonitrile with a slight excess of caustic (NaOH) in anaqueous system. The presence of the caustic in the present system is ofno major significance with respect to the overall principles ofoperation, the major effect being that of producing a reduction in thesolubility of NTA in water. In general, the feed may range from asaturated solution of NTA upwards to more concentrated systems includingsolutions containing more than a saturation amount of NTA and, downwardon the other hand, to solutions containing 5 percent or less (total) NTAby weight. In general, one prefers to pre-concentrate extremely diluteNTA solutions in a preceding evaporator to maintain a fairly constanttypical concentration of NTA in the feed ranging from about 30 to about40 percent NTA by weight.

Effluent is removed from the system at any convenient point, typicallyfrom the lines 19 or 20 or directly from the liquid of flash chamber 10.For simplicity and ease of control, the rate of withdrawal of theproduct is governed by the rate required to maintain a constant levelwithin flash chamber 10 as sensed and regulated by level control 28.

Product removed through line 27 contains on a weight percent basis fromabout 30 to about 35 percent NTA crystals and from about to aboutpercent mother liquor, the mother liquor containing-at least asaturation amount of NTA of about 33 percent, plus about 5 percentsodium hydroxide and about 62 percent water.

The feed and effluent rates are typically proportioned, together withthe liquid volume of the system, to provide an average NTA particleretention time in the system of about one hour with a convenient averagecirculation through pump 21 of about 30 times. Although this volume ispreferred for convenience and economy of construction, suitable resultsare obtained with the average residence time ranging from about 5minutes to about 10 hours with average circulation ranging from about 5to about times.

Although the two-pump scheme of the Figure is preferred because of theability to separately control the circulations for evaporation and forcrystal size control, it is possible to operate with a singlecentrifugal or turbine" pump rather than with two pumps. The approach tothe problem in such instances is to design for the impeller peripheralspeed indicated for crystal size control and to use an impeller vanewidth which provides an acceptable flow rate for acceptable heat input.

Although the flow through a separate heater 17 is generally preferred ina crystallization system to minimize fouling of heat exchanger surfacesand obtain good heat transfer, in appropriate instances the use of aseparate heat exchanger may be 'unnecessary in which instance the pump16 and the separate heater 17 are omitted. Where such is the case it ispreferred to use the crystal control loop with its separate pump 21designed for flow rates and peripheral speeds as set forth in theforegoing. Alternately, the centrifugal or turbine" impeller of pump 21may be inserted directly into the liquid mass in chamber 10 which may ormay not operate as a flash chamber. In this instance, the impel ler isoperated at a peripheral speed as indicated in the foregoing and thethickness or diameter of the impeller or impellers is proportioned toachieve the desired control. In such instances, a first approximationfor design is to use the same horsepower input for operation of thedirect impeller as used in operation of a separate pump 21.

The following examples indicate ments of the present invention.

EXAMPLE I Apparatus was set up as in the Figure but without pump 21.Pump 16 was a centrifugal pump with a diameter of 11% inches operativeat 1,160 rpm providing a peripheral velocity of 53 feet per second. Thetemperature of the flash chamber 10 was l8l.SF. The heatup throughheater 17 was 35F. The discharge through valve 27 contained 32 wt.percent NTA particles. The feed rate provided an average solids (NTAcrystals) residence time of 0.85 hours.

The product centrifuged well, providing recovered NTA crystals[N(CI-I,COONa),-I-I,O] of a bulk density of 0.68 containing 6-8 wt.percent moisture.

EXAMPLE II Example I was repeated using a pump speed of 795 rpmproviding a peripheral velocity of 40 feet per second. The temperatureof the flash chamber 10 was 180.5F, the heatup through heater 17 was3.0F. The discharge through valve 27 contained 27.6 wt. percent NTAparticles. The feed rate provided an average solids residence time of1.08 hours.

The product centrifuged well providing recovered NTA crystals of a bulkdensity of 0.66 containing 6-8 wt. percent moisture.

EXAMPLE III Example I was repeated using a pump speed of 795 rpmproviding a peripheral velocity of 40 feet per second. The temperatureof flash chamber 10 was 196F, the heatup through heater 17 was 3.5F. Thedischarge through valve 27 contained 32.4 weight percent NTA particles.The feed rate provided an average solids residence time of 1.2 hours.

The product centrifuged well providing recovered NTA crystals of a bulkdensity of 0.65 containing 4 percent water.

preferred embodi- EXAMPLE IV Example I was repeated using a pump speedof 795 rpm providing a peripheral velocity of 40 fps. The temperature ofthe flash chamber was 195F, the heatup through heater 17 was 3F. Thedischarge through valve 27 contained 27.6 wt. percent NTA particles. Thefeed rate provided an average solids residence time (NTA) of 1.25 hours.

The product centrifuged providing recovered NTA crystals of a bulkdensity of 0.6 containing 6-9 wt. percent moisture.

EXAMPLE V Example 1 was repeated using a pump speed of 635 rpm providinga peripheral velocity of 32 fps. The temperature of the flash chamber 10was 161F, the heatup through heater 17 was 4.0F. The discharge throughvalve 27 contained 27.0 wt. percent NTA particles. The feed rateprovided an average solids (NTA particles) residence time of 1.2 hours.

The product centrifuged but not as well as in Examples l-IV. RecoveredNTA had a bulk density of 0.55 and contained 10-12 wt. percent moisture.

EXAMPLE V1 Example I was repeated using a pump speed of 450 rpmproviding a peripheral velocity of 22 feet per second. The temperatureof the flash chamber was 183F, the heatup through heater 17 was 46F. Thedischarge through valve 27 contained 24.2 wt. percent NTA particles. Thefeed rate provided an average solids (NTA particles) residence time of0.9 hours.

The product would not centrifuge.

A sample of the product was obtained by filtration and other procedures.The bulk density was 0.5.

EXAMPLE V11 Foregoing examples were repeated with a centrifugal pumphaving a 4% inch diameter impeller operative at 2,700 rpm providing 53feet per second peripheral velocity. Desirable results similar to thoseof Examples l-IV were obtained.

EXAMPLE V111 Foregoing examples were repeated in several examples with acentrifugal pump having a 10% inch diameter impeller operative at 785,1,160 and 1,900 rpm, providing, respectively, 36, 53, and 87 feet persecond peripheral velocity.

Operation at 53 feet per second provided superior results with respectto ease of centrifuging, thoroughness of centrifuging, and bulk densityof product.

EXAMPLE IX Apparatus was set up as in the Figure, with pumps 16 and 21.Pump 21 discharged to the junction of lines 18 and 19. The liquidcontent of the system was 13 gallons. The feed rate was 162 lbs/hour.The product rate was 1 19 lbs/hours (slurry containing 30 percent byweight of NTA crystals) providing an average particle residence time inthe system of 1.3 hours.

Pump 16 was a centrifugal pump having 21.13 inch diameter impellerdriven at a speed of 369 rpm providing a peripheral velocity of 21 feetper second. The flow through this pump was 18 gals./min., approximately140 percent of the system volume per minute.

Pump 21 was a centrifugal pump having a 6 inch diameter impeller drivenat a speed of 1,875 rpm providing a peripheral velocity of 49 feet persecond. The

flow through this pump was 7 gallons per minute, approximately 54percent of the system content per minute.

The product from the system centrifuged readily providing a particulateNTA product having a bulk density of 38.7 lbs./ft with the followingdistribution:

3 percent on 30 mesh 42 percent on 50 mesh percent on 100 mesh 98percent on 200 mesh EXAMPLE X Example [X was repeated except pump 21 wasinoperative. Other flow rates, through pump 16, feed and product, werethe same. The product centrifuged poorly providing a product bulkdensity of 32.9 lbs/ft with the following distribution:

15 percent on 30 mesh 72 percent on 50 mesh percent on 100 mesh 98.5percent on 200 mesh We claim:

1. In a process for producing solid particulate NTA of high bulk densityby crystallization in a system containing growing crystals of NTA in asupersaturated aqueous solution of NTA, the improvement wherein saidsolution and the crystals of NTA contained therein are circulatedthrough a centrifugal impeller operative at a peripheral velocity offrom about 20 to about 80 feet per second whereby the crystals areselectively abraded or fractured to limit growth of the periphery of thecrystals enhancing the relative growth rate of the crystals in thethickness dimension.

2. The improvement of claim 1 wherein the temperature is from about toabout 250F, and wherein the centrifugal impeller is the impeller of acentrifugal pump.

3. A process for producing solid particulate NTA of high bulk densitywhich comprises, forming a system containing a mixture of growingcrystals of NTA and an accompanying supersaturated aqueous mother liquorsolution, said system containing from about 5 to about 50 percent byweight of crystals of NTA, mechanically agitating said system while thecrystals are growing by circulation thereof through a centrifugalimpeller providing a moderate degree of shear operation at a peripheralvelocity of from about 20 to about 80 feet per second whereby thecrystals formed are selectively abraded. v v

4. The process of claim 3 wherein the centrifugal impeller peripheralvelocity is from about 40 to about 70 feet per second.

5. The process of claim 3 wherein the centrifugal impeller peripheralvelocity is from about 45 to about 55 feet per second.

6. The-process of claim 3 wherein the centrifugal impeller is in a pumphousing and the entire volume of said system circulates through the pumphousing in a period of from about 0.5 to about 5 minutes.

7. The process of claim 6 wherein the circulation of the entire systemthrough the pump housing is an average of about once every 2 minutes.

8. The process of claim 3 wherein the solution temperature is from about120 to about 250F.

9. The process of claim 3 wherein the solution temperature is from aboutto about 200F.

10. The process of claim 3 wherein the solution temperature is fromabout 175 to about 195C.

11. The process of claim 3 wherein the solution temperature is about 195F.

12. The process of claim 3 wherein the system contains from about toabout 40 wt. percent NTA crystals.

13. The process of claim 3 wherein the system contains about 35 wt.percent NTA crystals.

14. A process for producing solid particulate NTA of high bulk densityand controlled particle size and shape wherein the NTA is crystallizedfrom a solution, which comprises:

recirculating a system consisting essentially of an aqueous solution ofNTA and from about 5 to about 50 wt. percent NTA crystals in anevaporator system wherein the system is alternately heated to atemperature of about 200F and flashed at a temperature of about Fwhereby water vapor is revolume of said system.

* i t i P0405) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent No. 5,758, 565 Dated September 11, 1975 Inven r) Y, Robert B.Agee and William R. Cooke It is certified that error appears in theabove-identified patent and that said Letters Patent arehereby'corrected as shown below:

Column line 7, reads "im epll should read impell- -j-. Column 10, line10, reads "sheat", should read shear Signed and sealed this 5th day ofMarch 197R.

(SEAL) Attest:

EDWARD M. FLETCHER, JR. C. MARSHALL DANN Attesting Officer Commissionerof Patents

2. The improvement of claim 1 wherein the temperature is from about 120* to about 250*F, and wherein the centrifugal impeller is the impeller of a centrifugal pump.
 3. A process for producing solid particulate NTA of high bulk density which comprises, forming a system containing a mixture of growing crystals of NTA and an accompanying supersaturated aqueous mother liquor solution, said system containing from about 5 to about 50 percent by weight of crystals of NTA, mechanically agitating said system while the crystals are growing by circulation thereof through a centrifugal impeller providing a moderate degree of shear operation at a peripheral velocity of from about 20 to about 80 feet per second whereby the crystals formed are selectively abraded.
 4. The process of claim 3 wherein the centrifugal impeller peripheral veLocity is from about 40 to about 70 feet per second.
 5. The process of claim 3 wherein the centrifugal impeller peripheral velocity is from about 45 to about 55 feet per second.
 6. The process of claim 3 wherein the centrifugal impeller is in a pump housing and the entire volume of said system circulates through the pump housing in a period of from about 0.5 to about 5 minutes.
 7. The process of claim 6 wherein the circulation of the entire system through the pump housing is an average of about once every 2 minutes.
 8. The process of claim 3 wherein the solution temperature is from about 120* to about 250*F.
 9. The process of claim 3 wherein the solution temperature is from about 150* to about 200*F.
 10. The process of claim 3 wherein the solution temperature is from about 175* to about 195*C.
 11. The process of claim 3 wherein the solution temperature is about 195* F.
 12. The process of claim 3 wherein the system contains from about 20 to about 40 wt. percent NTA crystals.
 13. The process of claim 3 wherein the system contains about 35 wt. percent NTA crystals.
 14. A process for producing solid particulate NTA of high bulk density and controlled particle size and shape wherein the NTA is crystallized from a solution, which comprises: recirculating a system consisting essentially of an aqueous solution of NTA and from about 5 to about 50 wt. percent NTA crystals in an evaporator system wherein the system is alternately heated to a temperature of about 200*F and flashed at a temperature of about 195*F whereby water vapor is removed, feeding make-up NTA solution to said system consisting essentially of from about 30 to about 40 wt. percent NTA and water, said recirculating being in two streams including (1) a low shear stream operative at a mass flow rate per minute of from about one to about 10 times the volume of said system and (2) a moderate sheat stream containing a centrifugal pump operative at an impeller peripheral velocity of from about 20 to about 80 feet per second and at a mass flow rate per minute of from about 0.2 to about twice the volume of said system. 